1. Field of the Invention
The invention relates to cathode ray tube displays and more particularly to electron beam focusing and power supply circuits for use particularly with such tubes having substantially flat viewing faces.
2. Description of the Prior Art
It is generally recognized by experts in the display art that the focal point of a cathode ray beam must coincide with the phosphor screen and that the distance the beam travels to that screen may vary considerably over the indicator face, especially in the instance of substantially flat-faced indicators. In addition, the focal point for the electron beam must be varied, again dynamically, as a function of brightness of the trace, or indirectly, as the average video input level.
With respect to the brightness function, an increase or decrease in the trace brightness is caused by an increase or decrease in electron beam current which, in turn, causes a space charge expansion or contraction of the electron beam diameter. Such variation therefore normally requires an adjustment of the electron beam focal point whenever the video input level is changed. This has been accomplished to a minor degree manually in the past, or automatically by employment of a small light sensor viewing the display face.
The focal distance change required for different deflections of the electron beam arises in the nature of the cathode ray tube face and the deflection coil geometry. In general, the curvature of the indicator viewing face does not coincide with the effective center of deflection characteristic of the deflection coils; without suitable compensation, elements of the displayed matter will be considerably blurred and distorted, especially at the periphery of the viewing face.